Recycled paper is increasingly used as a pulp source. One of the major pulping steps involves removal of the ink from any source of printed recycled paper. Large volumes of water are required for the ink removal process and its clean-up is accomplished using a solids/liquid separation unit operation. Dissolved air flotation (DAF) is commonly used.
Dissolved air flotation is a solids-removal process where fine air bubbles become attached to the suspended particles, thus reducing the density of individual particles and causing them to float to the surface. The separated solids then form a floating layer that is a mixture of solids and air bubbles. The buoyant force exerted by the entrapped air also acts to compact the solids into a smaller volume before the floating layer is skimmed off.
Air is usually introduced to the inflowing ink-laden water via a pressurized mixing chamber. When the influent enters the non-pressurized flotation unit, the supersaturated solution releases the air in the form of very fine bubbles which become attached to the suspended particles. To get maximum results from a flotation unit, a clarification aid must be added along with the air. Flotation methods can achieve high levels of suspended solids removal, up to 98%.
Recycle mills are most frequently located in the metropolitan areas where an emphasis on closing the water cycle of the mill is great. Consequently, effective clean-up of the deinking wash waters becomes important because reuse of the water generated e.g. from a DAF, can lead to reduced sheet quality such as brightness. Also, if these waters are used for other purposes, minimizing the amounts of BOD/-COD and suspended solids is desirable.
Clarification chemicals are typically utilized in conjunction with mechanical clarifiers for the removal of solids from the process water stream. The clarification chemicals coagulate and/or flocculate the suspended solids into larger particles, which can then be removed from the process stream by gravitational settling or flotation.
Clarification generally refers to the removal of nonsettleable material by coagulation, flocculation and sedimentation. Coagulation is the process of destabilization by charge neutralization. Once neutralized, particles no longer repel each other and can be brought together. Coagulation is necessary for removal of colloidal sized suspended matter. Flocculation is the process of bringing together the destabilized, "coagulated" particles to form a larger agglomeration or floc.
Depending upon the characteristics of the individual waters, differing chemical types and programs may be utilized. It is conventional to utilize a dual polymer program for clarification of deinking process waters by dissolved air flotation. Typically, this comprises a low molecular weight cationic coagulant followed by a high molecular weight flocculant.
It is customary to use coagulants prior to using flocculants in the deinking process waters to provide charge neutralization. This affords efficient solids removal. Typical conventional cationic coagulants are poly(diallyldimethyl ammonium chloride), amphoteric diallyldimethyl ammonium chloride/acrylic acid containing copolymers, condensation polymers of ethylene dichloride/ammonia or diinethylamine/epichlorohydrin. Conventional acrylamide-based flocculants have been utilized to assist in the solid/liquid separation. For example, novel copolymers of poly(diallyldimethylammonium chloride/3-acrylamido-3-methylbutanoic acid) are disclosed in U.S. Pat. No. 5,207,924, and polymers formed from vinylamine are disclosed in U.S. Pat. No. 5,573,675 as coagulants for the clarification of deinking process waters. Moreover, the use of hydrophilic dispersion polymers for the clarification of deinking process waters are disclosed in U.S. patent application Ser. No. 08/743,437, now U.S. Pat. No. 5,750,034, the disclosure of which is hereby incorporated by reference. In that reference, the dispersion polymer must be utilized in a dual polymer program for optimal activity.
By contrast, within the general class of polymers described herein, the preferred polymer of the invention is a hydrophilic dispersion copolymer of diallyldimethyl ammonium chloride (DADMAC) and (meth)acrylamide--a single treatment agent. The advantage of the invention disclosed herein is that it is a method which employs a dispersion polymer which is the sole treatment agent. Though other treatment agents may be added as adjuncts, they are not required for activity. Another advantage of this invention is that the use of these dispersion polymers affords removal of particulate materials without the unwanted addition of oils and surfactants contained in conventional latex polymers. Moreover, these polymers require no inverter system and can be introduced to the paper process stream using simple feeding equipment.